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University of Montana

Abstract

The smaller clutch size of tropical as opposed to north temperate birds has intrigued researchers for a long time. An untested hypothesis posits that higher nest predation in the tropics favors smaller nests thereby constraining clutch-size. We tested this hypothesis by conducting an experiment to test whether nest predation increases with nest size in a tropical forest. Furthermore, we studied north temperate and tropical birds to examine if: (1) predation rates increased with nest size, (2) nest sizes were smaller in the tropics, and (3) clutch size was explained by nest size controlled for body size. We used data on predation rates, nest sizes, and clutch sizes for > 2000 north temperate and tropical bird nests of 36 altricial bird species that nest in open cups. Nest predation risk increased with nest size in both the experiment and in the comparison across latitudes, justifying a major premise underlying the nest size hypothesis. However, nest sizes were not smaller in the tropics. As a result, clutch sizes were not related to nest sizes either between latitudes or within sites. Nest sizes were strongly correlated with adult body sizes. Hence, (1) body size might influence reproductive success by affecting nest predation through nest size; and (2) we rejected the hypothesis that nest size explains clutch size in the tropics.